1 Field of the Invention
This invention relates generally to devices to control power distribution. More particularly, this invention relates to a three-phase, dc-to-ac power inverter that utilizes three-level pole valves to produce waveforms with zero voltage components.
2 Background Information
Three-phase, dc-to-ac power inverters are used in electric power distribution systems. These devices have a set of switches that are used to convert a dc voltage signal into discreetly displaced square waveforms. The waveforms are subsequently combined to produce a high quality sinusoidal output signal.
FIG. 1 illustrates a prior art three-phase, dc-to-ac power inverter 20. The inverter 20 includes a first inverter stage 22 and a second inverter stage 24 connected by a dc source 26. Each inverter stage includes twelve two-level inverter poles 28.
FIG. 2a is an enlarged view of a two-level inverter pole 28. Each two-level inverter pole 28 includes a positive polarity thyristor 30 and an anti-parallel diode 32. When the positive polarity thyristor 30 is fired (closed), a positive dc waveform is produced at the output node (Vout). Each two-level inverter pole 28 also includes a negative polarity thyristor 34 and an anti-parallel diode 36. When the negative polarity thyristor 34 is fired, a negative dc waveform is produced at the output node. FIG. 2b illustrates a two-level square wave 38 produced by the two-level inverter pole 28. The term "two-level" is used in reference to the signal because the signal either has a positive value (V/2) or a negative value (-V/2).
Returning now to FIG. 1, it can be appreciated that the different two-level inverter poles 28 are used to generate a set of square waveforms. The waveforms are then combined by a set of interphase transformers 40. In addition to combining waveforms, the interphase transformers 40 serve to remove harmonic components associated with the input waveforms. The outputs of the interphase transformers are combined at a harmonic blocking transformer 42. The output from the harmonic block transformer 42 is applied to a main transformer 44, which includes primary delta windings 45, primary wye windings 46, and secondary delta windings 48. The three-phase output of the main transformer 44 is then applied to a load 50 which may be an electric power transmission line in the case of a utility application.
FIG. 3 illustrates a single phase, forty-eight pulse waveform generated by the apparatus of 20 of FIG. 1. Each pulsed signal is generated by a two-level inverter pole 28. By phase-shifting the signals generated at the two-level inverter poles 28 and then combining the phase-shifted signals with the transformers 40, 42 and 44, the waveform of FIG. 3 is produced. The output of the main transformer 44 includes the signal of FIG. 3 along with two identical signals which are phase-shifted 120.degree. from one another.
In many existing applications, it is sufficient for an inverter to have limited control capability. For example, limited control capability is sufficient in static condensers (STATCONs). A static condenser is a power circuit that is connected in shunt with the power line to draw a controlled reactive current, thereby regulating the voltage at the point of connection and increasing the achievable power transmission. In existing static condensers, the inverter is controlled entirely by varying the phase angle of the inverter output voltage. There is no direct control of the ratio between the dc input voltage and the ac output voltage. Accordingly, the inverter cannot, for example, be controlled to produce a desired mix of positive and negative sequence voltages at its terminals. In addition, the dc input voltage cannot be maintained at a substantially constant level while varying the ac output voltage.
New applications are emerging where it is important for an inverter to quickly generate an arbitrary output voltage vector. That is, it is important for the inverter to quickly generate arbitrary magnitude and phase voltage quantities.
An example of the need for arbitrary inverter output voltage vector control is the unified power flow controller described in U.S. Pat. No. 5,343,139 (the '139 patent). The '139 patent, which is expressly incorporated by reference herein, describes an apparatus with an inverter serially connected (serial inverter) to a three-phase distribution network, an inverter connected in parallel (parallel inverter) to the three-phase distribution network, and a common dc source supplying each inverter. The dc voltage is held substantially constant by the parallel inverter, but the series inverter must produce widely varying ac voltage. In this case, a fast vector-controlled inverter makes it possible to implement active feedback control of the transmission line power which would not otherwise be possible. Another demanding application for a fast-acting vector-controlled inverter is flicker-reduction on power lines supplying electric arc furnaces.
Thus, it would be highly desirable to provide an improved three-phase, dc-to-ac power inverter. More particularly, it would be highly desirable to provide a three-phase, dc-to-ac power inverter that can rapidly generate arbitrary magnitude and phase output voltage values.